To optimize the thermal efficiency of large scale fossil fuel burning heat exchangers or boilers, it is necessary to periodically remove deposits such as soot, slag and fly ash from their interior heat exchanging surfaces. Typically, a number and types of cleaning devices known as sootblowers are mounted to the exterior of the boiler. Periodically they are inserted into the boiler through cleaning ports located in the boiler wall. Positioned on the forward end of the screw tubes or screw tubes assemblies are one or more cleaning nozzles. The nozzles discharge a pressurized fluid cleaning medium, such as air, water, or steam. The high pressure cleaning medium causes deposits of soot, slag, and fly ash to be dislodged from the internal structures of the boiler.
One type of sootblower is known as a short travel retracting rotary type. This type has a screw tube assembly which is inserted into the boiler, and once it reaches its fully extended position, cleaning medium is discharged from the nozzle as it is rotated through a partial arc, full rotation, or multiple full rotations as desired for wall cleaning. The sootblower medium discharged from the nozzle provides the cleaning effect mentioned previously. One very widely utilized design of the above-mentioned sootblower type is manufactured by the assignee of the present invention and is known as a Diamond Power “IR-3Z”™ sootblower device. These devices have operated in a highly reliable and effective manner around the world for many years.
One disadvantage of many sootblower designs is the erosion and thermal stresses caused to internal components of the boiler when their cleaning cycle operates in the same repeated manner during each operation. For the sootblower of the type mentioned previously, once the screw tube assembly is advanced and reaches its fully extended position, the nozzle begins to discharge cleaning medium and rotates through a specified arc or number of revolutions. At the conclusion of the cleaning cycle, the nozzle reaches its set rotational indexed position, at which point the screw tube assembly is retracted. The next operating cycle retraces the path of the prior cycles. When steam is used as a sootblowing medium, steam in the supply circuit piping may condense into liquid water between operating cycles. When the steam valve is opened to cause the steam sootblowing medium to flow through the sootblower at the beginning of a cleaning cycle, an initial pulse of condensate is ejected from the sootblower nozzle. Thereafter, high pressure steam flows through the nozzle until the cleaning medium valve is again shut-off. The initial ejection of the condensate has an undesirable consequence of placing erosion and thermal stresses on the internal components which impacts it. The heat transfer surfaces can tolerate condensate, but when numerous cycles occur in which the same surfaces are repeatedly impacted by condensate, failures of the internal heat transfer components can occur. Accordingly, in many applications it is desirable to index the position at which the sootblower nozzle begins its cleaning cycle so that the same internal surfaces are not struck by condensate at the start of each operating cycle.
Numerous approaches toward providing sootblower nozzle indexing are known. For example, in long retracting sootblowers which discharge cleaning medium as a lance tube is extended and retracted, the cleaning medium path can be displaced between operating cycles. An approach implemented by the assignee of this invention for indexing long retracting sootblowers uses a drive rack for a gear driven type long retracting sootblower which features a mechanism for indexing the phasing of gear drive between operating cycles. This approach is described in the assignee's U.S. Pat. No. 4,803,959. Other types of indexing mechanisms are known, for example, some use gear drives having ratcheting indexing components.
While many approaches toward providing indexing of sootblower operating cycles are known, these strategies are not adaptable for modification to existing short travel retracting rotary sootblowers.
In accordance with the present invention, these inventors have found that modifications of the existing IR-3Z™ sootblower components coupled with modifications of the control schedule of the device provide the desirable indexing feature. By preferably providing at least four different rotated start positions for the sootblowing start cycle, the erosion effects of condensate ejection can be distributed over multiple internal surfaces, reducing the likelihood of boiler component damage. The principles of this invention may be implemented as a modification to existing sootblowers or in newly constructed sootblower assemblies.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.